I was going to get back to you and explain further why brass is a bettercartridge case material than steel or aluminum. Sorry I took so long. Ileft you with the nebulous comment that brass was "stretchier" and wouldspring back more so it was easier to extract from the chamber after firing.Now I'll attempt to show why this is true given the basic material propertieslisted above.

A synopsis would be that the propellant pressure expands the diameter ofthe thin wall of the cartridge case until it contacts the interior wallof the chamber and thereafter it expands the case and the chambertogether. The expansion of the cartridge case, however, is not elastic.The case is enough smaller in diameter than the chamber that it has to_yield_ to expand to chamber diameter. After the pressure is relieved bythe departure of the bullet, both the chamber and the cartridge casecontract elastically. It is highly desirable that the cartridge casecontract more than the chamber so that the case may be extracted with aminimum of effort.

A quick review of the Young's modulus: this is sort of the "springconstant" of a material; it is the inverse of how much a unit chunk ofmaterial stretches under a unit load. Its units are stress / strain =psi/(inch/inch). Here's a basic example of its use: If you have a 2inch by 2 inch square bar of steel which is 10 inches long and you put a10,000 pound load on it, how much does it stretch? First of all, thestress on the steel is 10,000/(2*2) = 2500 psi. The strain per inch willbe 2500 psi/29*10^6 = 0.000086 inches/inch. So the stretch of a 10 inchlong bar under this load will be 10 * 0.000086 = 0.00086 inches or alittle less than 1/1000 inch.

Yield stress (aka yield strength) is the load per unit area at which amaterial starts to yield or take a permanent set (git bint). It's notan exact number because materials often start to yield slightly and thengo gradually into full-scale yield. But the transition is fast enoughto give us a useful number.

So how far can you stretch CDA 260 cartridge brass before it takes apermanent set? That would be yield stress divided by Young's modulus:63,000 psi/16*10^6 psi/(inch/inch) = .004 inches/inch.

How far can you stretch cheap steel? Try A36 structural steel:36,000 psi/29*10^6 psi/(inch/inch) = .001 inches/inch.How about good steel of modest cost such as C1118?77,000 psi/29*10^6 psi/(inch/inch) = .003 inches/inch.(Note that C1118 doesn't have anywhere near the formability of CDA 260.Brass cases are made by the cheap forming process called "drawing"while C1118 is a machinable steel, suitable for the more expensive machiningprocesses such as turning and milling.)

What about something that's expensive such as CDA 172 beryllium copper?175,000 psi/19*10^6 psi/(inch/inch) = .009 inches/inch.(This isn't serious because CDA 172 is pretty brittle when it's _this_hard. Not to mention that beryllium is poisonous even in an alloyed state.)

Titanium Ti-6AL-4V150,000 psi/16.5*10^6 psi/(inch/inch) = .009 inches/inch(This is an excellent material though expensive and hard to work with.)

Really expensive aluminum, 7075-T673,000 psi/10.4*10^6 psi/(inch/inch) = .007 inches/inchCheap aluminum, 3003 H1829,000 psi/10*10^6 psi/(inch/inch) = .003 inches/inch(Aluminum isn't a really good material because it isn't strong and cheapat the same time, it hasn't much fatigue strength, and it won't go overits yield stress very often without breaking. So you can't reload it.It makes a "one-shot" case at best. Also, 7075 is a machinable ratherthan a formable aluminum, primarily.)

+Here's the important part: Even if you stretch something until it+yields, it still springs back some distance. In fact, the springback+amount is the same as if you had just barely taken the thing up to its+yield stress. This is because when you stretch it, you establish a newlength for it, and since you are holding it at the yield stress (atleast until you release the load) it will spring back the distanceassociated with that yield stress. So the figures given above such as.004 inches/inch are the figures that tell us how much a case springsback after firing.

Changing subjects for a moment: How much does the steel chamber expandand contract during a firing? Naturally this amount is partiallydetermined by the chamber wall's thickness. The outside diameter of arifle chamber is about 2 1/2 times the maximum inside diameter,typically. The inside diameter is around .48 inches at its largest.Actual chamber pressures of high pressure rounds will run 60,000 psi oreven 70,000 psi range if you're not careful.

One of the best reference books on the subject is "Formulas for Stressand Strain" by Roark and Young, published by MacGraw-Hill. Everyonejust calls it "Roark's". In the 5th edition, example numbers 1a & 1b,page 504, I find the following:

A rifle's chamber is capped at one end and open at the other but reallyit's not too open at the other end because the case is usually bottle-necked. You'd have to go back to basics instead of using cookbookformulae if you wanted the exact picture, but if we compute the resultsof both formulas, the truth must lie between them but closer to thecapped vessel.

There's not a whole heck of a lot of difference between the two resultsso let's just say that the chamber's expansion is .001 inch radial or.002 inch diametral.

The cartridge case's outside diameter is equal to about .48 inch afterthe cartridge has been fired. So its springback, if made from CDA 260,is .004 inches/inch (from above) * .48 inch = .002 inches diametralwhich of course is just the amount the chamber contracted so we've justbarely got an extractable case when chamber pressures hit 70,000 psi inthis barrel. This is why the ease with which a case can be extractedfrom a chamber is such a good clue as to when you are reaching maximumallowable pressures. By the same token, you can see that if a chamber'swalls are particularly thin, it will be hard to extract cases (regardlessof whether or not these thin chamber walls are within their stress limits).A really good illustration of this can be found when comparing the S&Wmodel 19 to the S&W model 27. Both guns are 357 magnum caliber and bothcan take full-pressure loads without bursting. The model 27 has thickchamber walls and the model 19 has thin chamber walls. Cartridge caseswhich contained full-pressure loads are easily extracted from a model 27but they have to be pounded out of a model 19. So manufacturers don'tmanufacture full-pressure loads for the 357 magnums anymore. 8-(

We can see from the above calculations that a steel case wouldn't be agood idea for a gun operating at 70,000 psi with the given 2.5:1 OD/IDchamber wall ratio if reasonable extraction force is a criterion. Lowerpressures and/or thicker chamber walls could allow the use of steel cases.